ABSTRACT/SUMMARY The Primary Immune Deficiency Treatment Consortium (PIDTC), an RDCRN consortium of 44 immunology and hematopoietic stem cell transplant centers throughout the USA and Canada, was established in 2009 to study rare genetic disorders of the immune system, collectively known as primary immunodeficiency diseases (PIDs). The goals of the PIDTC are to understand PIDs and define optimal approaches for their definitive treatment. In its first 9 years, the PIDTC has studied outcomes following hematopoietic cell transplantation (HCT), gene therapy (GT) and enzyme replacement therapy (ERT) for patients with severe combined immunodeficiency (SCID), Wiskott-Aldrich syndrome (WAS) and chronic granulomatous disease (CGD). These PIDs were chosen because they have been among the most life-threatening and difficult to treat, often requiring HCT for survival. Because no single center follows enough affected individuals to encompass the full spectrum of these disorders, a consortium is essential to define the natural history of each PID. Moreover, historically, individual centers developed their own approaches to treatment, without consensus regarding indications or timing for HCT, types of conditioning regimens, or sources of donor cells. Thus, multicenter studies are required for robust statistical assessment to compare impacts not only of patient-related variables, but also of treatment-related variables on clinical outcome. The PIDTC is organized to develop, perform and learn from multicenter studies. Our major contributions to understanding of PID pathogenesis and defining which treatments produce optimal clinical outcomes have been published in 111 papers. Close relationships with multiple Patient Advocacy Groups (PAGs) have led to publications on high-priority issues for affected individuals and their families, including quality of life and long-term outcomes. PIDTC Pilot Projects have advanced newborn screening for SCID and introduced important mechanistic studies, while our Career Enhancement Core has held an annual PIDTC Scientific Workshop and Education Day and has supported 20 PID trainees, all of whom remain active in academic research. PIDTC studies of SCID have enabled our design and implementation of the first prospective multicenter trial to determine the minimal dose of busulfan conditioning to achieve T and B cell immune reconstitution. Looking forward, the PIDTC will undertake a new initiative to study Primary Immune Regulatory Disorders (PIRD) as it continues its studies of SCID and CGD. The major impact of the proposed research will be establishment of baseline data and organizational structures to undertake multicenter clinical trials that will apply improved basic understanding to achieve further evidence-based advances in the care of PIDs.